The production of liquid crystal displays such as, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled. The downdraw sheet drawing processes and, in particular, the fusion process described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capable of producing glass sheets that can be used as substrates without requiring costly post-forming finishing operations such as lapping and polishing. Unfortunately, the fusion process places rather severe restrictions on the glass properties, which require relatively high liquidus viscosities.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred in certain applications because of their ability to transport electrons more effectively. Poly-crystalline (p-Si) based silicon transistors are characterized as having a higher mobility than those based on amorphous-silicon (a-Si) based transistors. This allows the manufacture of smaller and faster transistors, which ultimately produces brighter and faster displays. In certain other applications, a layer of single-crystalline semiconductor material such as silicon formed on the surface of the glass substrate is even more desirable because the even higher performance of the semiconducting device components based on single-crystalline silicon.
One problem with p-Si and single-crystalline silicon based transistors is that their manufacture requires higher process temperatures than those employed in the manufacture of a-Si transistors. There are multiple steps with temperatures ranging from 450° C. to 750° C. for p-Si, and even higher for single-crystalline silicon, compared to the 400° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction. Compaction, also referred to as thermal stability or dimensional change, is an irreversible dimensional change (shrinkage) in the glass substrate due to changes in the fictive temperature of the glass material. “Fictive temperature” is a concept used to indicate the structural state of a glass. Glass that is cooled quickly from a high-temperature is said to have a higher fictive temperature because of the “frozen in” higher temperature lower density structure. Glass that is cooled more slowly, or that is annealed by holding for a time near its annealing point, is said to have a lower fictive temperature and higher density structure.
The magnitude of compaction depends both on the process by which a glass is made and the viscoelastic properties of the glass. In the float process for producing sheet products from glass, the glass sheet is cooled relatively slowly from the melt and, thus, “freezes in” a comparatively low-temperature structure into the glass. The fusion process, by contrast, results in very rapid quenching of the glass sheet from the melt, and freezes in a comparatively high-temperature structure. As a result, a glass produced by the float process may undergo less compaction when compared to glass produced by the fusion process, since the driving force for compaction is the difference between the fictive temperature and the process temperature experienced by the glass during compaction. Thus, it would be desirable to minimize the level of compaction in a glass substrate that is produced by a downdraw process.
There are two approaches to minimizing compaction in glass. The first is to thermally pretreat the glass to create a fictive temperature similar to the one the glass will experience during the p-Si TFT manufacture. However there are several difficulties with this approach. First, the multiple heating steps employed during the p-Si TFT manufacture create slightly different fictive temperatures in the glass that cannot be fully compensated for by this pretreatment. Second, the thermal stability of the glass becomes closely linked to the details of the p-Si TFT manufacturer, which could mean different pretreatments for different end-users. Finally, pretreatment adds to processing costs and complexity.
Another approach is to slow the kinetics of the compaction response. This can be accomplished by raising the viscosity of the glass. Thus, if the strain point of the glass is much greater than the process temperatures to be encountered (e.g., if the strain point is ˜50-100° C. greater than the process temperatures for short exposures), compaction is minimal. The challenge with this approach, however, is the production of high strain point glass that is cost effective because higher strain point glasses normally require higher melting temperatures.
Hence, there remains a need of a glass material possessing high strain points and, thus, good dimensional stability (i.e., low compaction) and with reasonable melting temperatures. Desirably, the glass compositions also possess all of the properties required for downdraw processing, including but not limited to fusion down-draw processing, which is important in the manufacturing of substrates for liquid crystal displays.
The present invention addresses this and other needs.